"True" iron(V) and iron(VI) porphyrins: a first theoretical exploration

Comparative oxidative ability of mononuclear and dinuclear high-valent iron-oxo species towards the activation of methane: does the axial/bridge atom modulate the reactivity?

Over the years, mononuclear Fe^IVO species have been extensively studied, but the presence of dinuclear Fe^IVO species in soluble methane monooxygenase (sMMO) has inspired the development of biomimic models that could activate inert substrates such as methane. There are some successful attempts; particularly the [(Por)(m-CBA) Fe^IV(μ-N)Fe^IV(O)(Por˙⁺)]^- species has been reported to activate methane and yield decent catalytic turnover numbers and therefore regarded as the closest to the sMMO enzyme functional model, as no mononuclear Fe^IVO analogues could achieve this feat. In this work, we have studied a series of mono and dinuclear models using DFT and ab initio DLPNO-CCSD(T) calculations to probe the importance of nuclearity in enhancing the reactivity. We have probed the catalytic activities of four complexes: [(HO)Fe^IV(O)(Por)]^- (1), [(HO)Fe^IV(O)(Por˙⁺)] (2), μ-oxo dinuclear iron species [(Por)(m-CBA)Fe^IV(μ-O)Fe^IV(O) (Por˙⁺)]^- (3) and N-bridged dinuclear iron species [(Por)(m-CBA)Fe^IV(μ-N)Fe^IV(O)(Por˙⁺)]^- (4) towards the activation of methane. Additionally, calculations were performed on the mononuclear models [(X)Fe^IV(O)(Por˙⁺)]ⁿ {X = N 4a (n = -2), NH 4b (n = -1) and NH₂4c (n = 0)} to understand the role of nuclearity in the reactivity. DFT calculations performed on species 1-4 suggest an interesting variation among them, with species 1-3 possessing an intermediate spin (S = 1) as a ground state and species 4 possessing a high-spin (S = 2) as a ground state. Furthermore, the two Fe^IV centres in species 3 and 4 are antiferromagnetically coupled, yielding a singlet state with a distinct difference in their electronic structure. On the other hand, species 2 exhibits a ferromagnetic coupling between the Fe^IV and the Por˙⁺ moiety. Our calculations suggest that the higher barriers for the C-H bond activation of methane and the rebound step for species 1 and 3 are very high in energy, rendering them unreactive towards methane, while species 2 and 4 have lower barriers, suggesting their reactivity towards methane. Studies on the system reveal that model 4a has multiple FeN bonds facilitating greater reactivity, whereas the other two models have longer Fe-N bonds and less radical character with steeper barriers. Strong electronic cooperativity is found to be facilitated by the bridging nitride atom, and this cooperativity is suppressed by substituents such as oxygen, rendering them inactive. Thus, our study unravels that apart from enhancing the nuclearity, bridging atoms that facilitate strong cooperation between the metals are required to activate very inert substrates such as methane, and our results are broadly in agreement with earlier experimental findings.

Visible light generation of high-valent metal-oxo intermediates and mechanistic insights into catalytic oxidations

High-valent metal-oxo complexes play central roles as active oxygen atom transfer (OAT) agents in many enzymatic and synthetic oxidation catalysis. This review focuses on our recent advances in application of photochemical approaches to probe the oxidizing metal-oxo species with different metals and macrocyclic ligands. Under visible light irradiation, a variety of important metal-oxo species including iron-oxo porphyrins, manganese-oxo porphyrin/corroles, ruthenium-oxo porphyrins, and chromium-oxo salens have been successfully generated. Kinetical studies in real time have provided mechanistic insights as to the reactivity and reaction pathways of the metal-oxo intermediates in their oxidation reactions. In photo-induced ligand cleavage reactions, metals in n⁺ oxidation state with the oxygen-containing ligands bromate, chlorate, or nitrites were photolyzed. Homolytic cleavage of the O-X bond in the ligand gives (n + 1)⁺ oxidation state metal-oxo species, and heterolytic cleavage gives (n + 2)⁺ oxidation state metal-oxo species. In photo-disproportionation reactions, reactive Mⁿ⁺¹-oxo species can be formed by photolysis of μ-oxo dimeric Mⁿ⁺ complexes with the concomitant formation of M^n-1 products. Importantly, the oxidation of M^n-1 products by molecular oxygen (O₂) to regenerate the μ-oxo dimeric Mⁿ⁺ complexes in photo-disproportionation reactions represents an attractive and green catalytic cycle for the development of photocatalytic aerobic oxidations.

Mechanistic insights into the allylic oxidation of aliphatic compounds by tetraamido iron(v) species: A C–H vs. O–H bond activation

This work is based on a deep insight into a comparative study of C–H vs. O–H bond activation of allylic compound by the high valent iron complex. Our theoretical findings can help to design catalysts with better efficiency for catalytic reactions.

Insights from kinetic studies of photo-generated compound II models: Reactivity toward aryl sulfides

Iron(IV)-oxo porphyrins [Fe^IV(Por)O] (Por = poprhyrin), commonly called compound II models, were produced in three electron-deficient ligands by visible light irradiation of highly photo-labile porphyrin-iron(III) bromates or chlorates. The kinetics of oxygen transfer atom (OAT) reactions with aryl sulfides by these photo-generated [Fe^IV(Por)O] (3) were studied in CH₃CN solutions. The iron(IV)-oxo porphyrins under study include 5,10,15,20-tetra(2,6-dichlorophenyl)porphyrin-iron(IV)-oxo (3a), 5,10,15,20-tetra(2,6-difluorophenyl)porphyrin-iron(IV)-oxo (3b), and 5,10,15,20-tetra(pentafluorophenyl)porphyrin-iron(IV)-oxo (3c). As expected, complexes 3 were competent oxidants and reacted rapidly with thioanisoles to give the corresponding sulfoxides with minor over-oxidation sulfones. Apparent second-order rate constants determined under pseudo-first-order conditions for sulfide oxidation reactions are (9.8 ± 0.1) × 10²-(3.7 ± 0.3) × 10¹ M^-1 s^-1, which are 3 to 4 orders of magnitude greater in comparison to those of alkene epoxidations and activated CH bond oxidations by the same oxo species. Conventional Hammett analyses gave non-linear correlations, indicating no significant charge developed at the sulfur during the oxidation process. For a given substrate, the reactivity order for the iron(IV)-oxo species was 3c < 3b < 3a, which is inverted from expectations on the basis of the electron-withdrawing capacity of the porphyrin macrocycles. The absolute rate constants from kinetic studies provided insights into the transient oxidants in catalytic reactions under turnover conditions where actual reactive intermediates are not observable. Our kinetic and catalytic competition results strongly suggest that 3 may undergo a disproportionation reaction to form a higher oxidized iron(IV)-oxo porphyrin radical cations as the true oxidant.

Electronic Structure of Corrole Derivatives: Insights from Molecular Structures, Spectroscopy, Electrochemistry, and Quantum Chemical Calculations

Presented herein is a comprehensive account of the electronic structure of corrole derivatives. Our knowledge in this area derives from a broad range of methods, including UV-vis-NIR absorption and MCD spectroscopies, single-crystal X-ray structure determination, vibrational spectroscopy, NMR and EPR spectroscopies, electrochemistry, X-ray absorption spectroscopy, and quantum chemical calculations, the latter including both density functional theory and ab initio multiconfigurational methods. The review is organized according to the Periodic Table, describing free-base and main-group element corrole derivatives, then transition-metal corroles, and finally f-block element corroles. Like porphyrins, corrole derivatives with a redox-inactive coordinated atom follow the Gouterman four-orbital model. A key difference from porphyrins is the much wider prevalence of noninnocent electronic structures as well as full-fledged corrole^•2- radicals among corrole derivatives. The most common orbital pathways mediating ligand noninnocence in transition-metal corroles are the metal(d_z2)-corrole("a_2u") interaction (most commonly observed in Mn and Fe corroles) and the metal(d_x2-y2)-corrole(a_2u) interaction in coinage metal corroles. Less commonly encountered is the metal(d_π)-corrole("a_1u") interaction, a unique feature of formal d⁵ metallocorroles. Corrole derivatives exhibit a rich array of optical properties, including substituent-sensitive Soret maxima indicative of ligand noninnocence, strong fluorescence in the case of lighter main-group element complexes, and room-temperature near-IR phosphorescence in the case of several 5d metal complexes. The review concludes with an attempt at identifying gaps in our current knowledge and potential future directions of electronic-structural research on corrole derivatives.

An investigation of ligand effects on the visible light-induced formation of porphyrin–iron(iv)-oxo intermediates

Depending on the structure of the porphyrin ligands, the visible light photolysis of porphyrin–iron(iii) bromates produced iron(iv)-oxo radical cations or iron(iv)-oxo porphyrins, permitting direct kinetic studies of their oxidation reactions.

Ligand control in the photochemical generation of high-valent porphyrin-iron-oxo derivatives

Visible-light irradiation of photo-labile bromate porphyrin-iron(III) salts gave iron(IV)-oxo porphyrin radical cations (compound I model) or the neutral iron(IV)-oxo porphyrin (compound II model), depending on the electronic structure of porphyrin ligands.

ortho-Hydroxylation of aromatic acids by a non-heme Fe(V)=O species: how important is the ligand design?

There is a growing interest in probing the mechanism of catalytic transformations effected by non-heme iron-oxo complexes as these reactions set a platform for understanding the relevant enzymatic reactions. The ortho-hydroxylation of aromatic compounds is one such reaction catalysed by iron-oxo complexes. Experimentally [Fe(II)(BPMEN)(CH3CN)2](2+) (1) and [Fe(II)(TPA)(CH3CN)2](2+) (2) (where TPA = tris(2-pyridylmethyl)amine and BPMEN = N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)ethane-1,2-diamine) complexes containing amino pyridine ligands along with H2O2 are employed to carry out these transformations where complex 1 is found to be more reactive than complex 2. Herein, using density functional methods employing B3LYP and dispersion corrected B3LYP (B3LYP-D) functionals, we have explored the mechanism of this reaction to reason out the importance of ligand design in fine-tuning the reactivity of such catalytic transformations. Dispersion corrected B3LYP is found to be superior to B3LYP in predicting the correct ground state of these species and also yields lower barrier heights than the B3LYP functional. Starting the reaction from the Fe(III)–OOH species, both homolytic and heterolytic cleavage of the O···O bond is explored leading to the formation of the transient Fe(IV)=O and Fe(V)=O species. For both the ligand systems, heterolytic cleavage was energetically preferable and our calculations suggest that both the reactions are catalyzed by an elusive high-valent Fe(V)=O species. The Fe(V)=O species undergoes the reaction via an electrophilic attack of the benzene ring to effect the ortho-hydroxylation reaction. The reactivity pattern observed for 1 and 2 are reflected in the computed barrier heights for the ortho-hydroxylation reaction. Electronic structure analysis reveals that the difference in reactivity between the ligand architectures described in complex 1 and 2 arise due to orientation of the pyridine ring(s) parallel or perpendicular to the Fe(V)=O bond. The parallel orientation of the pyridine ring is found to mix with the (πFe(dyz)–O(py))* orbital of the Fe-oxo bond leading to a reduction in the electrophilicity of the ferryl oxygen atom. Our calculations highlight the importance of ligand design in this chemistry and suggest that this concept can be used to (i) stabilize high-valent intermediates which can be trapped and thoroughly characterized (ii) enhance the reactivity and efficiency of the oxidants by increasing the electrophilicity of the ferryl oxygen containing FeVO species. Our computed results are in general agreement with the experimental results.

A convenient procedure for the synthesis of fluoro-iron(III) complexes of common synthetic porphyrinates

We report here an improved method for the preparation of fluoro-iron(III) porphyrinate complexes. Treatment of [ Fe 2( P )2(μ- O )] (P = tetraphenylporphyrinate {TPP}, tetra-p-tolylporphyrinate {TTP}, or octaethylporphyrinate {OEP}) or [ Fe ( OH )( OH 2)( TMP )] (TMP = tetramesitylporphyrinate) with the commercially available fluorinating agent, Et 3 N ·3 HF , in dichloromethane affords the desired [ FeF ( P )] complexes in a straightforward fashion and in good yield while avoiding the use of aqueous hydrofluoric acid. All fluoro-iron(III) complexes have been completely characterized by a series of different spectroscopic techniques including cyclic voltammetry. Reaction of a representative complex, [ FeF ( OEP )], with various chloride reagents demonstrates that halide exchange with chloride is facile, but only proceeds at an appreciable rate in the presence of proton sources. Unexpectedly, treatment of [ FeF ( OEP )] with NOBF 4 did not to lead formation of an oxidized species, but rather to formation of the { Fe – NO }6 complex, [ Fe ( NO )( OEP )]( BF 4).

The protein effect in the structure of two ferryl-oxo intermediates at the same oxidation level in the heme copper binuclear center of cytochrome c oxidase

Identification of the intermediates and determination of their structures in the reduction of dioxygen to water by cytochrome c oxidase (CcO) are particularly important to understanding both O2 activation and proton pumping by the enzyme. In this work, we report the products of the rapid reaction of O2 with the mixed valence form (CuA(2+), heme a(3+), heme a3(2+)-CuB(1+)) of the enzyme. The resonance Raman results show the formation of two ferryl-oxo species with characteristic Fe(IV)=O stretching modes at 790 and 804 cm(-1) at the peroxy oxidation level (PM). Density functional theory calculations show that the protein environment of the proximal H-bonded His-411 determines the strength of the distal Fe(IV)=O bond. In contrast to previous proposals, the PM intermediate is also formed in the reaction of Y167F with O2. These results suggest that in the fully reduced enzyme, the proton pumping ν(Fe(IV)=O) = 804 cm(-1) to ν(Fe(IV)=O) = 790 cm(-1) transition (P→F, where P is peroxy and F is ferryl) is triggered not only by electron transfer from heme a to heme a3 but also by the formation of the H-bonded form of the His-411-Fe(IV)=O conformer in the proximal site of heme a3. The implications of these results with respect to the role of an O=Fe(IV)-His-411-H-bonded form to the ring A propionate of heme a3-Asp-399-H2O site and, thus, to the exit/output proton channel (H2O) pool during the proton pumping P→F transition are discussed. We propose that the environment proximal to the heme a3 controls the spectroscopic properties of the ferryl intermediates in cytochrome oxidases.

Heme cytotoxicity and the pathogenesis of immune-mediated inflammatory diseases

Heme, iron (Fe) protoporphyrin IX, functions as a prosthetic group in a range of hemoproteins essential to support life under aerobic conditions. The Fe contained within the prosthetic heme groups of these hemoproteins can catalyze the production of reactive oxygen species. Presumably for this reason, heme must be sequestered within those hemoproteins, thereby shielding the reactivity of its Fe-heme. However, under pathologic conditions associated with oxidative stress, some hemoproteins can release their prosthetic heme groups. While this heme is not necessarily damaging per se, it becomes highly cytotoxic in the presence of a range of inflammatory mediators such as tumor necrosis factor. This can lead to tissue damage and, as such, exacerbate the pathologic outcome of several immune-mediated inflammatory conditions. Presumably, targeting “free heme” may be used as a therapeutic intervention against these diseases.

Production of a putative iron(V)-oxocorrole species by photo-disproportionation of a bis-corrole-diiron(IV)-mu-oxo dimer: implication for a green oxidation catalyst

Photodisproportionation of a bis-corrole-diiron(IV)-mu-oxo dimer gave a corrole-iron(III) species and a corrole-iron(V)-oxo species that can be detected and studied in real time. Air oxidation of the corrole-iron(III) species regenerated the bis-corrole-diiron(IV)-mu-oxo dimer, allowing the development of a photocatalytic method for organic oxidations using molecular oxygen and visible light.

Highly reactive porphyrin-iron-oxo derivatives produced by photolyses of metastable porphyrin-iron(IV) diperchlorates

Photolyses of metastable porphyrin-iron(IV) diperchlorates in laser flash photolysis reactions gave highly reactive transients. The systems studied were 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetramesitylporphyrin (TMP), and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP). The new species, which decayed within milliseconds in acetonitrile solutions, were shown to react with organic substrates by oxo-transfer reactions involving insertions into carbon-carbon double bonds of alkenes and styrenes or benzylic carbon-hydrogen bonds of arenes. The order of reactivity was OEP > TPP > TMP. Second-order rate constants for reactions with several substrates at 22 degrees C were determined; representative values of rate constants for the TPP derivative were k = 8.6 x 10(5) M(-1) s(-1) for styrene, k = 2.5 x 10(6) M(-1) s(-1) for cyclohexene, and k = 7.7 x 10(4) M(-1) s(-1) for ethylbenzene. These porphyrin-iron-oxo transients reacted 4-5 orders of magnitude faster than the corresponding iron(IV)-oxo porphyrin radical cations with rate constants similar to those of porphyrin-manganese(V)-oxo derivatives. Rate constants for oxidations of benzylic C-H positions of arenes correlated with the C-H bond dissociation energies, and Hammett correlations for reactions with substituted styrenes had rho(+) values ranging from -0.5 to -0.7, reflecting electrophilic character of the oxidants and their high reactivity. On the basis of their unique UV-visible spectra, high reactivities, and oxo-transfer properties, the new transients are tentatively identified as porphyrin-iron(V)-oxo perchlorates, electronic isomers (or valence tautomers) of well-known iron(IV)-oxo porphyrin radical cations.

Formation of stable and metastable porphyrin- and corrole-iron(IV) complexes and isomerizations to iron(III) macrocycle radical cations

Oxidations of three porphyrin-iron(III) complexes (1) with ferric perchlorate, Fe(ClO(4))(3), in acetonitrile solutions at -40 degrees C gave metastable porphyrin-iron(IV) diperchlorate complexes (2) that isomerized to known iron(III) diperchlorate porphyrin radical cations (3) when the solutions were warmed to room temperature. The 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetramesitylporphyrin (TMP), and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) systems were studied by UV-visible spectroscopy. Low temperature NMR spectroscopy and effective magnetic moment measurements were possible with the TPP and TMP iron(IV) complexes. Reactions of two corrole systems, 5,10,15-tris(pentafluorophenyl)corrole (TPFC) and 5,15-bis(pentafluorophenyl)-10-p-methoxyphenylcorrole (BPFMC), also were studied. The corrole-iron(IV) chlorides reacted with silver salts to give corrole-iron(IV) complexes. The corrole-iron(IV) nitrate complexes were stable at room temperature. (TPFC)-iron(IV) toslyate, (TPFC)-iron(IV) chlorate, and (BPFMC)-iron(IV) chlorate were metastable and rearranged to their electronic isomers iron(III) corrole radical cations at room temperature. (TPFC)-iron(III) perchlorate corrole radical cation was the only product observed from reaction of the corrole-iron(IV) chloride with silver perchlorate. For the metastable iron(IV) species, the rates of isomerizations to the iron(III) macrocycle radical cation electronic isomers in dilute acetonitrile solutions were relatively insensitive to electron demands of the macrocyclic ligand but reflected the binding strength of the ligand to iron. Kinetic studies at varying temperatures and concentrations indicated that the mechanisms of the isomerization reactions are complex, involving mixed order reactivity.

Density functional theory study of trans-dioxo complexes of iron, ruthenium, and osmium with saturated amine ligands, trans-[M(O)2(NH3)2(NMeH2)2]2+ (M=Fe, Ru, Os), and detection of [Fe(qpy)(O)2]n+ (n=1, 2) by high-resolution ESI mass spectrometry

Density functional theory (DFT) calculations on trans-dioxo metal complexes containing saturated amine ligands, trans-[M(O)2(NH3)2(NMeH2)2]2+ (M=Fe, Ru, Os), were performed with different types of density functionals (DFs): 1) pure generalized gradient approximations (pure GGAs): PW91, BP86, and OLYP; 2) meta-GGAs: VSXC and HCTH407; and 3) hybrid DFs: B3LYP and PBE1PBE. With pure GGAs and meta-GGAs, a singlet d2 ground state for trans-[Fe(O)2(NH3)2(NMeH2)2]2+ was obtained, but a quintet ground state was predicted by the hybrid DFs B3LYP and PBE1PBE. The lowest transition energies in water were calculated to be at lambda approximately 509 and 515 nm in the respective ground-state geometries from PW91 and B3LYP calculations. The nature of this transition is dependent on the DFs used: a ligand-to-metal charge-transfer (LMCT) transition with PW91, but a pi(Fe-O)-->pi*(Fe-O) transition with B3LYP, in which pi and pi* are the bonding and antibonding combinations between the dpi(Fe) and ppi(O(2-)) orbitals. The FeVI/V reduction potential of trans-[Fe(O)2(NH3)2NMeH2)2]2+ was estimated to be +1.30 V versus NHE based on PW91 results. The [Fe(qpy)(O)2](n+) (qpy=2,2':6',2'':6'',2''':6''',2''''-quinquepyridine; n=1 and 2) ions, tentatively assigned to dioxo iron(V) and dioxo iron(VI), respectively, were detected in the gas phase by high-resolution ESI-MS spectroscopy.

Laser flash photolysis generation of high-valent transition metal-oxo species: insights from kinetic studies in real time

High-valenttransition metal-oxo species are active oxidizing species in many metal-catalyzed oxidation reactions in both Nature and the laboratory. In homogeneous catalytic oxidations, a transition metal catalyst is oxidized to a metal-oxo species by a sacrificial oxidant, and the activated transition metal-oxo intermediate oxidizes substrates. Mechanistic studies of these oxidizing species can provide insights for understanding commercially important catalytic oxidations and the oxidants in cytochrome P450 enzymes. In many cases, however, the transition metal oxidants are so reactive that they do not accumulate to detectable levels in mixing experiments, which have millisecond mixing times, and successful generation and direct spectroscopic characterization of these highly reactive transients remain a considerable challenge. Our strategy for understanding homogeneous catalysis intermediates employs photochemical generation of the transients with spectroscopic detection on time scales as short as nanoseconds and direct kinetic studies of their reactions with substrates by laser flash photolysis (LFP) methods. This Account describes studies of high-valent manganese- and iron-oxo intermediates. Irradiation of porphyrin-manganese(III) nitrates and chlorates or corrole-manganese(IV) chlorates resulted in homolytic cleavage of the O-X bonds in the ligands, whereas irradiation of porphyrin-manganese(III) perchlorates resulted in heterolytic cleavage of O-Cl bonds to give porphyrin-manganese(V)-oxo cations. Similar reactions of corrole- and porphyrin-iron(IV) complexes gave highly reactive transients that were tentatively identified as macrocyclic ligand-iron(V)-oxo species. Kinetic studies demonstrated high reactivity of the manganese(V)-oxo species, and even higher reactivities of the putative iron(V)-oxo transients. For example, second-order rate constants for oxidations of cis-cyclooctene at room temperature were 6 x 10(3) M(-1) s(-1) for a corrole-iron(V)-oxo species and 1.6 x 10(6) M(-1) s(-1) for the putative tetramesitylporphyrin-iron(V)-oxo perchlorate species. The latter rate constant is 25,000 times larger than that for oxidation of cis-cyclooctene by iron(IV)-oxo perchlorate tetramesitylporphyrin radical cation, which is the thermodynamically favored electronic isomer of the putative iron(V)-oxo species. The LFP-determined rate constants can be used to implicate the transient oxidants in catalytic reactions under turnover conditions where high-valent species are not observable. Similarly, the observed reactivities of the putative porphyrin-iron(V)-oxo species might explain the unusually high reactivity of oxidants produced in the cytochrome P450 enzymes, heme-thiolate enzymes that are capable of oxidizing unactivated carbon-hydrogen bonds in substrates so rapidly that iron-oxo intermediates have not been detected under physiological conditions.

Probing the Compound I-like reactivity of a bare high-valent oxo iron porphyrin complex: the oxidation of tertiary amines

The mechanisms of oxidative N-dealkylation of amines by heme enzymes including peroxidases and cytochromes P450 and by functional models for the active Compound I species have long been studied. A debated issue has concerned in particular the character of the primary step initiating the oxidation sequence, either a hydrogen atom transfer (HAT) or an electron transfer (ET) event, facing problems such as the possible contribution of multiple oxidants and complex environmental effects. In the present study, an oxo iron(IV) porphyrin radical cation intermediate 1, [(TPFPP)*+ Fe(IV)=O]+ (TPFPP = meso-tetrakis (pentafluorophenyl)porphinato dianion), functional model of Compound I, has been produced as a bare species. The gas-phase reaction with amines (A) studied by ESI-FT-ICR mass spectrometry has revealed for the first time the elementary steps and the ionic intermediates involved in the oxidative activation. Ionic products are formed involving ET (A*+, the amine radical cation), formal hydride transfer (HT) from the amine ([A(-H)]+, an iminium ion), and oxygen atom transfer (OAT) to the amine (A(O), likely a carbinolamine product), whereas an ionic product involving a net initial HAT event is never observed. The reaction appears to be initiated by an ET event for the majority of the tested amines which included tertiary aliphatic and aromatic amines as well as a cyclic and a secondary amine. For a series of N,N-dimethylanilines the reaction efficiency for the ET activated pathways was found to correlate with the ionization energy of the amine. A stepwise pathway accounts for the C-H bond activation resulting in the formal HT product, namely a primary ET process forming A*+, which is deprotonated at the alpha-C-H bond forming an N-methyl-N-arylaminomethyl radical, A(-H)*, readily oxidized to the iminium ion, [A(-H)]+. The kinetic isotope effect (KIE) for proton transfer (PT) increases as the acidity of the amine radical cation increases and the PT reaction to the base, the ferryl group of (TPFPP)Fe(IV)=O, approaches thermoneutrality. The ET reaction displayed by 1 with gaseous N,N-dimethylaniline finds a counterpart in the ET reactivity of FeO+, reportedly a potent oxidant in the gas phase, and with the barrierless ET process for a model (P)*+ Fe(IV)=O species (where P is the porphine dianion) as found by theoretical calculations. Finally, the remarkable OAT reactivity of 1 with C6F5N(CH3)2 may hint to a mechanism along a route of diverse spin multiplicity.

Photochemical production of a highly reactive porphyrin-iron-oxo species

Oxidation of 5,10,15,20-tetramesitylporphyrinatoiron(III) perchlorate, (TMP)FeIII(ClO4), with ferric perchlorate in acetonitrile gave a metastable species identified as (TMP)FeIV(ClO4)2 that decayed within seconds to the known isomeric species (TMP*+)FeIII(ClO4)2. Irradiation of the metastable species with 355 nm laser light gave a highly reactive transient that reacts with simple organic reductants (alkenes and arylalkanes) 5 orders of magnitude faster than known Compound I analogues, (TMP*+)FeIV(O)(X-).

Transition metal spin state energetics and noninnocent systems: challenges for DFT in the bioinorganic arena

Although density functional theory (DFT) provides a generally good description of transition metal systems, we have identified several cases, involving Fe(III) porphyrins and related systems, where common functionals fail to correctly describe the energetics of the different low-lying spin states. The question of metal- versus ligand-centered oxidation in high-valent transition metal complexes is also a challenging one for DFT calculations, as I have tried to illustrate with examples from among porphyrin, corrole, biliverdine, and NO complexes. In a number of cases, I have compared results obtained with different exchange-correlation functionals; in addition, I have added a discussion on the relative performance of pure versus hybrid functionals. Finally, I have offered some thoughts on the role that traditional wavefunction-based ab initio methods, now essentially absent from the bioinorganic arena, might play in the future.

Cytochrome p450 compound I

Cytochrome P450 enzymes (P450s) comprise a large class of enzymes that effect numerous oxidations in nature. The active oxidants in P450s are thought to be iron(IV)-oxo porphyrin radical cations termed Compounds I, and these intermediates have been sought since the discovery of P450s 40 years ago. We report formation of the Compound I derivative of a P450 enzyme by laser flash photolysis oxidation of the corresponding Compound II species, an iron(IV)-oxo neutral porphyrin intermediate. The Compound II derivative in turn was produced by oxidation of the P450 with peroxynitrite, which effected a net one-electron, oxo-transfer reaction to the iron(III) atom of the resting enzyme. For the P450 studied in this work, CYP119 from the thermophile Sulfolobus solfactaricus, the P450 Compound II derivative was stable for seconds at ambient temperature, and the Compound I transient decayed with a lifetime of ca. 200 ms.

Kinetic studies of reactions of iron(IV)-oxo porphyrin radical cations with organic reductants

Iron(IV)-oxo porphyrin radical cations are observed intermediates in peroxidase and catalase enzymes, where they are known as Compound I species, and the putative oxidizing species in cytochrome P450 enzymes. In this work, we report kinetic studies of reactions of iron(IV)-oxo porphyrin radical cations that can be compared to reactions of other metal-oxo species. The iron(IV)-oxo radical cations studied were those produced from 5,10,15,20-tetramesitylporphryinato-iron(III) perchlorate (1), 5,10,15,20-tetramesitylporphryinato-iron(III) chloride (2), both in CH(3)CN solvent, and that from 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato-iron(III) perchlorate (3) in CH(2)Cl(2) solvent. The substrates studied were alkenes and activated hydrocarbons diphenylmethane and ethylbenzene. For a given organic reductant, various iron(IV)-oxo porphyrin radical cations react in a relatively narrow kinetic range; typically the second-order rate constants vary by less than 1 order of magnitude for the oxidants studied here and the related oxidant 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato-iron(IV)-oxo porphyrin radical cation in CH(3)CN solvent. Charge transfer in the transition states for epoxidation reactions of substituted styrenes by oxidants 1 and 2, rho(+) values of -1.9 and -0.9, respectively, mirrors results found previously for related species. Competition kinetic reactions with a catalytic amount of porphyrin iron(III) species and a terminal oxidant give relative rate constants for oxidations of competing substrates that are somewhat smaller than the ratios of absolute rate constants. Water in CH(3)CN solutions has an apparent modest stabilizing effect on oxidant 1 as indicated in slightly reduced rate constants for oxidation reactions. The iron(IV)-oxo porphyrin radical cations are orders of magnitude less reactive than porphyrin-manganese(V)-oxo cations and a corrole-iron(V)-oxo species. The small environment effects found here suggest that high energy demanding hydrocarbon oxidation reactions catalyzed by cytochrome P450 enzymes might require highly reactive iron(V)-oxo transients as oxidants instead of the more stable, isomeric iron(IV)-oxo porphyrin radical cations.

Discovery of a reaction intermediate of aliphatic aldoxime dehydratase involving heme as an active center

Recently, we discovered an intriguing hemoprotein [aliphatic aldoxime dehydratase (OxdA)] that catalyzes the dehydration of aliphatic aldoximes [R-CH=N-OH] to the corresponding nitriles [R-C identical withN] in the industrial Pseudomonas chlororaphis B23 strain. Unlike the utilization of H(2)O(2) or O(2) as a mediator of the catalysis by other heme-containing enzymes (e.g., P450), OxdA is notable for the direct binding of a substrate to the heme iron, experimental evidence of which was obtained here by means of resonance Raman (RR) analysis with an isotope technique. We found that the addition of a large amount of butyraldoxime (final concentration, 200 mM) to ferrous OxdA with a low enzyme concentration (final concentration, 5 muM) yields a long-lived OxdA-substrate complex (named OS-II), whose UV-vis spectrum is different from the corresponding spectra of the OxdA-substrate complex I and CO-bound, ferrous, and ferric forms of OxdA. Intriguingly, the RR analysis demonstrated that OS-II includes a highly oxidized heme with strong bonding between a substrate and the heme iron, as judged from the heme oxidation state marker nu(4) band at 1,379 cm(-1) and the (15)N-isotope-substituted butyraldoxime sensitive band at 857 cm(-1) in the RR spectra. It is noteworthy that OS-II has a highly oxidized heme like the ferryl-oxo heme species (e.g., compound II) formed by some general hemoproteins, although the function of OxdA is different from those (transport of electrons, transport of oxygen, sensing of oxygen or carbon monoxide, and catalysis of redox reactions) of general hemoproteins.

Highly reactive electrophilic oxidants in cytochrome P450 catalysis

The cytochrome P450 enzymes effect a wide range of oxidations in nature including difficult hydroxylation reactions of unactivated C-H. Most of the high energy reactions of these catalysts appear to involve highly electrophilic active species. Attempts to detect the reactive transients in the enzymes have met with limited success, but evidence has accumulated that two distinct electrophilic oxidants are produced in the P450 enzymes. The consensus electrophilic oxidant termed "iron-oxo" is usually thought to be an analogue of Compound I, an iron(IV)-oxo porphyrin radical cation species, but it is possible that a higher energy electronic isomer of Compound I is required to account for the facility of the C-H oxidation reactions. The second electrophilic oxidant of P450 is speculative; circumstantial evidence suggests that this species is iron-complexed hydrogen peroxide, but this oxidant might be a second spin state of iron-oxo. This overview discusses recent studies directed at detection of the electrophilic oxidants in P450 enzymes and the accumulated evidence for two distinct species.

Photochemical Generation of a Highly Reactive Iron−Oxo Intermediate. A True Iron(V)−Oxo Species?

Laser flash photolysis of 5,10,15-tris(pentafluorophenyl)corrole-iron(IV) chlorate or nitrate, prepared from the corresponding chloride, gave a highly reactive iron-oxo transient identified as an iron(V)-oxo species on the basis of its UV-visible spectrum and high reactivity as well as by analogy to photochemical ligand cleavage reactions of related manganese species. The transient was shown to be an oxo transfer agent in a preparative reaction with cis-cyclooctene. Representative rate constants for oxidation reactions by the new transient at ambient temperature were k = 5900 M-1 s-1 for cyclooctene and k = 570 M-1 s-1 for ethylbenzene. The new transient is more than 6 orders of magnitude more reactive with typical organic reductants than expected for an iron(IV)-oxo corrole radical cation and 100 times more reactive than an analogous positively charged iron(IV)-oxo porphyrin radical cation. Slow electron transfer isomerization of ligand iron(V)-oxo species to iron(IV)-oxo ligand radical cations might be important in reactions of porphyrin-iron catalysts in the laboratory and in nature.

Molecular modeling of the intercalation of porphyrins into α-zirconium phosphate

In this work, n-alkylamines (number of carbon atoms ranging from 3 to 10) were investigated in detail by molecular modeling as spacers for intercalating porphyrins into alpha-zirconium phosphate (alpha-ZrP). Pre-intercalated n-alkylamines can form either a flat monolayer or a canted bilayer in the gallery of alpha-ZrP. Based on the interlayer state and intercalative potential of the two modes in alpha-ZrP, it is suggested that the flat monolayer is a better spacer than the bilayer and that n-propylamine (PA) and n-butylamine (BA) in mobile monolayers are the best spacers among the n-alkylamines studied, as is also found experimentally. The intercalation behavior of TMPyP [5,10,15,20-tetrakis (1-methylpyridinium-4-yl) porphyrin] and several other porphyrins was investigated by calculating the intercalative potential. The calculated results showed that the porphyrins were densely packed in a canted monolayer model, and an increase of polarity of the substituent would facilitate the intercalation of the porphyrins.

Oxidizing intermediates in cytochrome P450 model reactions

Oxoiron(IV) porphyrin pi-cation radicals have been considered as the sole reactive species in the catalytic oxidation of organic substrates by cytochromes P450 and their iron porphyrin models over the past two decades. Recent studies from several laboratories, however, have provided experimental evidence that multiple oxidizing species are involved in the oxygen transfer reactions and that the mechanism of oxygen transfer is much more complex than initially believed. In this Commentary, reactive intermediates that have been shown or proposed to be involved in iron porphyrin complex-catalyzed oxidation reactions are reviewed. Particularly, the current controversy on the oxoiron(IV) porphyrin pi-cation radical as a sole reactive species versus the involvement of multiple oxidizing species in oxygen transfer reactions is discussed.